In conventional methods of manufacturing LEDs that emit white light, a slurry in which at least one kind of phosphor such as YAG, TAG, or silica-based material and a binder are mixed is dispensed onto an ultraviolet or blue light emitting diode to coat it, a like slurry further containing a solvent added to reduce the viscosity is directly sprayed to the LED using a spray device as a kind of fine particle generating device to coat it, a phosphor plate is prepared to cover the LED, or a phosphor sheet called a remote phosphor is prepared and provided at a location remote from the LED.
Patent Document 1 discloses a method for manufacturing an LED by applying a slurry containing a phosphor to a heated LED chip by spraying while whirling the slurry with compressed air, thereby applying the slurry to a side surface of the LED, which is considered to be difficult to coat by common spraying methods.
Patent Document 2 discloses a process of coating an LED chip with a binder such as silicone and curing it, applying a slurry composed of a phosphor, a binder, and a solvent on it, and laminating a diffuser with them in a mixed manner when necessary.
Patent Document 3 discloses a process of transferring a slurry composed of a phosphor, a binder, and a solvent having a viscosity between 0.1 and 200 cps between two syringes, applying teaching of Japanese Patent Application Laid-Open No. 2004-300000, and applying the slurry to a chip multiple times while whirling the spray stream employing an air pulse spray taught by Japanese Patent Application Laid-Open No. 59-281013
A method using a dispenser as disclosed in Non-Patent Document 1 is widely employed to fill a cup in which a chip is mounted with the slurry for mass production of artillery-shell-shaped LEDs of not high power and LEDs for back light.
It is true that the method disclosed in Patent Document 1 increases chances of arrival of slurry particles to the side surface by whirling of the spray stream. However, to achieve a color temperature of approximately 5000K, it is necessary to provide a coating having a dried slurry weight per unit area of 20 to 100 micrometers in equivalent film thickness, which may vary in relation to the ratio of the phosphor and the binder.
To achieve a color temperature of approximately 2700K, it is necessary to add a red-tinged phosphor and to approximately double the coating thickness, namely to provide a coating having a thickness of 40 to 200 micrometers, and in the case where the slurry is diluted, the wet coating thickness needs to be 1.5 or 2 times thicker. Then, even if heated, a temporal decrease in the viscosity will cause the separation of the coating from the top end face and side surface of the chip, making it impossible to provide a coating having a desired thickness.
In the method disclosed in Patent Document 2, a binder is applied to an LED chip and cured, and then a slurry containing a phosphor is applied thereon by air spraying. However, it is common knowledge among engineers involved in spray coating and in the field of the art that it is impossible to coat a side surface of an LED having corners with sprayed particles at a desired thickness by common air spraying, because the volume of air is 400 to 600 times the volume of sprayed particles and the air arriving at the corners of the LED acts like a cushion to push back ceaselessly-coming air containing particles in a repeated manner.
In the method disclosed in Patent Document 3, while the quality of covering of edges and wall faces tends to be improved by application of coating in multiple thin layers each having a thickness of 3 to 10 micrometers, the chip is heated at low temperature typically in the range of 40° C. to 80° C. in order to prevent unevenness in coating thickness due to bumping of solvent vapor at the moment of spray coating or generation of pin holes, which might be caused if the chip is heated excessively. However, cross linking of a binder such as a silicone is not promoted at high speed at such temperatures. Then, the binder dissolves in the solvent again or swells to cause sinking of the coating at positions near edges and/or flow of coating. Thus, ideal coating cannot be achieved. For this reason, the object to be coated is taken out of the coating apparatus every time at least one layer of coating is applied and dried in a separate drying device for several minutes at a temperature in the range of 150° C. to 200° C. to promote gelation.
Furthermore, a metal masking is placed on the portions of a ceramic substrate or wafer level LED as an object to be coated to which coating should not be applied, in some cases. In such cases, coating on masking plate is removed after applying processing for assisting the removal, and the masking is once detached and attached again in order to prevent curing. In consequence, the time taken by the indirect work including the above-described steps is three to ten times the total coating time, leading to very low productivity.
On the other hand, in the case where a slurry without solvent containing a binder such as silicone and a phosphor is applied through a dispenser using a simple apparatus like one disclosed in Non-Patent Document 1, masking is not needed, and high productivity can be achieved. However, the LED chip is relatively thick or high in the central portion and thin in the edge portion as shown in FIG. 7, and therefore not only the vertical light distribution but also the spatial uniformity distribution thereof is not good. Therefore, this LED chip is not suitable for use as a high power LED for illumination purpose.